R1:c0ae2f628973

Show change diffs in Phriction

Summary:
This is really rough and needs work (particularly, there's some diff code I
really need to refactor since I sort-of-copy-pasted it) but basically
functional.

Show text changes between diffs and allow users to revert to earlier versions.

Differential's line-oriented diff style isn't ideal for large blocks of text but
I'm betting this is probably good enough in most cases. We can see how bad it is
in practice and then fix it if needbe.

I added a bunch of support…